Band-pass amplifier circuits



1 3- N. M. RUST EfAL 0 BAND-PASS AMPLIFIER CIRCUITS Filed April 30, 194p 1 INVENT NOEL M. RUST JOSEPH D. BRA/[$1 0M AND 51w ORS BY EiZPZOODENOU 6/1 ATTORNEY Patented Feb. 9, 1943 BAND-PASS AIWPLIFIER CIRCUITS UNITED STATES PATENT OFFICE Application April 30, 1941, Serial No. 391,204 In Great Britain October 19, 1939 7 Claims.

The present invention relates to band pass amplifier circuits and has for its principal object to provide a stable reaction whereby very sharp tuning may be obtained which is practically independent of valve adjustments.

According to the invention a multi-valve stage and an inter-valve coupling stage includes in the cathode leg or lead common to the input and output circuits of each valve a negative feedback circuit such that the product of the impedance of the feed-back circuit and of the mutual conductance of the valve with which it is associated .is considerably greater than unity, the wholearrangement being such as to produce a negative impedance effect which is independent of valve constants.

The invention will be described in connectio with the accompanying drawing in which Fig. 1 is a schematic circuit serving to explain the invention; Fig. 2 is an amplifier circuit according to the invention employing common cathode positive feed-back; Fig. 3 are curves showing the gain of the amplifier of Fig. 2 with change ingrid bias of the two valves; Figs. 4a to 40 are equivalent circuits which may be utilized in the circuit of Fig.2 to counteract the dephasingeffect of the grid-cathode capacity of the valves; and Fig. 5 shows the application of the present invention to a circuit utilizing a directly heated valve.

Considering Figure 1, it may be shown that the input admittance is given by !]AgB 11 t +gA I +gB m) IfZi and Zn: are each made equal to zero, that is if theybe absent, there will result the familiar equation YIN If, on the other hand, cathode back coupling be. introduced to the extent that the terms g Z: and gBZIII are each 1; the expression becomes -'.gAgB II ""ZII' m Z1 apprommately Thus by the choice of suitable values for Z1 and Zm in relation to QA and 9B it is possible to'produce a negative admittance at the input terminals-of a valve which is dependent upon the impedances Z1, Zn, and Zm and practically independent of the valve constants 9A and gB. When Zxand Z111 are circuits of very low Q, it will be seen that the variation of Ym over a band of frequencies will be practically dependand Zn be made equal to lcZ Znr and Z1 being It" will be seen thus that compensation of a parallel tuned circuit connected across the input terminals of the network can be produced by means of three parallel resonant circuit and by the correct choice of values, the effect of valve constants can'be eliminated.

Cathode back coupling a's'labo'v'e described can be app'lie'd'to' a variety of circuits. By correctly relating Z1, ZnandZm, and using diiierent types of circuits, a variety of correction effects can be obtained, and the scope of applications is not limited to radio frequencies; e. g. variousfornis of tone correction arrangements can be devised.

Again the cathode back coupling arrangements canbe a'pplied't'o' obtaining stable oscillators'in which the frequency is independent to a' large extent of valve and supply'variations.

It has been found difficult in experimental Work to secure stable operation of the crystal reversed impedance circuits. It i believed, however, that stabilisation by cathode back coupling should effect improvement. It should be understood that although for convenience of analysis cathode back coupling has been con sidered, other forms of back coupling may also be employed effectively.

Figure 2 illustrates an amplifier circuit wit common cathodepositive' feed back. Each valve is separately stabilised by an impedance in its cathode lead.

The gain of such an amplifier is given by the expression where m is the effective mutual conductance of either valve and is equal to 9/ (1+9 Z11) assuming ZlI=Zm.

This may be rewritten as i. e. Z having added to it in efiect a parallel negaand thus approaches a maximum value as g is increased. This is clearly shown by the curves of Figure 3 which show gain in decibels against grid bias of the two valves.

When the amplifier was set to oscillate by slightly increasing Zrv considerable improvement in the stability of frequency of oscillation with change of the H. T; voltage was noticeable, when compared with an unstabilized oscillator. In this amplifier, the cathode circuits Zn 211:, Zrv, were of very low Q, but were tuned to a somewhat lower frequency than that of the circuit Z1, which was 455 kc. This was to counteract the dephasing effect of the grid-cathode capacity of the valve.

The grid cathode capacity may be tuned out by coupling the coils tuning the grid and cath-' ode circuits. This may be accomplished in various ways, such as shown in Figs. 4a, 4b and 4c in which the coils only are shown coupled by the grid-cathode capacity C, and mutual inductance M. Coil L1 of Fig. 4a may correspond for example to the coil of Z1 in Fig. 2 and coil L2 of Fig. 4a may correspond for example to the coil of ZIII in Fig. 2.

' These coils L1 and L2 are coupled by a positive mutual inductance M as in Figs. 4a and 4b, and in Fig. 4c the coupling i adjusted so that LA tunes with C at the working frequency, the net coupling between LB and Lo then being zero.

The correct coupling is easily found in the case of Fig. 4a by tuning the coils L1 and L2 with condensers, and adjusting the coupling until the double hump response measured at the grid of the valve becomes a single hump, i. e. until the voltage, measured by a separate valve voltmeter, becomes a maximum, showing that the two tuned circuit are effectively uncoupled. The valve concerned should be rendered inoperative to avoid complication due to feed back eiiects.

The anode-cathode capacity may be similarly tuned out by coupling inductances in the anode lead and cathode lead.

It may also be shown that compensation for the effects of the grid/cathode capacity so far as feed back from the cathode circuit is concerned, may be obtained by this means. This may from cathode to grid is via a four-terminal network, such as a tapered line.

The use of a cathode stabilising impedance has been found to render a single valve reaction circuit stable and reliable, especially if a high a value be used. Greater stability of oscillation with variation of H. T. voltage than for the twovalve arrangement was also obtained with this circuit, when made to oscillate.

In a directly heated valve, the use 01"; a cathode circuit i not permissible for the provision of negative feed-back for stabilising purposes. The circuit described below has been devised to overcome this dimculty when used in conjunction with a positive feed back arrangement.

Figure shows the circuit or a reacting detector valve, where Z: is the ordinary tuned circult to which reaction is applied by a coupling be useful in cases, for example, where feed back coil from the anode. The earth end of this coil is connected to the junction of the impedances Zr and Zn where Zn takes the place of the oathode circuit hitherto considered. The current in the anode circuit is in antiphase with that in the grid circuit, so that the voltage developed across Zn will be in antiphase to that developed across Zr+Z'n by the input current. This negative feed back tends to eli'ect the positive feed-back provided to Z1 by the coupling coil, the coupling of which must therefore be tightened. As in the other circuits considered, any change in the valve that changes the positive feed-back, i. e..the

equivalent negative impedance shunting Z1, also changes the negative feed-back by the same amount and the net change is thereby reduced.

The coils tuning Z1 and Zn are coupled by a.

- positive mutual inductance to tune out stray capacities shunting them. Z'n, be of low Q.

What We claim is:

1. A multi-valve stage comprising a pair of valves, an inter-valve coupling impedance therebetween, and an impedance in the common part moreover, may

of the output and input circuits of each valve for producing negative feed-back from the output to the input of each valve, the value of each cathode impedance being such that the product of the impedance. and the mutual conductance of the valve with which it is associated is considerably greater than unity, the arrangement being such as to produce a negative admittance efiect at the input of the stage which is independent of valve constants.

2. A multi-valve stage as claimed in claim 1 wherein the cathode returns of said valves have a common portion in which there is included an impedance network for providing positive feed back from the output to the input of the stage.

3. A circuit arrangement in accordancev with claim 1 wherein the cathode returns of said valves have a common portion in which there is included an impedance network for providing positive feed-back from the output to the input of the stage, the value of the impedance of the said positive feed back network being such as to cause the said circuit arrangement to'oscillate.

4. A circuit arrangement as claimed in claim 1 wherein the inter-valve coupling impedance comprises a parallel tuned circuit, the negative feedback impedance in the common part of the output and input circuits of second valve also comprising a parallel tuned circuit, and wherein the grid cathode capacity of said valve is tuned out cults.

5. A circuit arrangement as claimed in claim 1 wherein the cathode returns of said valves have a common portion in which there is included an impedance network for providing positive feedback from the output to the input of the stage, the negative feed-back impedances and the positive feed back network are constituted by circuits of low Q value and are tuned to a lower frequency than the inter-valve coupling impedance.

6. A multi-valve stage comprising a first valve A having in the common part of the output and input circuits thereof a negative feed-back circuit having an impedance value Z1 and in the output circuit thereof an impedance value Zn, and a second valve 13 having in the common part of the output and input circuits thereof a negative feed-back circuit having an impedance value Zm, and wherein gAZ1 and 932m are each 1 whereby the input admittance Ym of the first valve is equal to yAQB I II' ZIZIII and wherein Z: and Zm in relation to 9A, the mutual conductance of valve A, and 9B, the mutual conductance of valve B, are so chosen as to produce a negative admittance at the input terminals of valve A which is dependent upon the impedances Z1, Zn, and Zn: and practically independent of the valve constants gA and 9B.

7. A circuit arrangement as claimed in claim 6 wherein Z: and Zm are circuits of very low Q whereby the variation of Ym over a band of frequencies will be practically dependent upon the variation of Zn.

NOEL MEYER RUST. JOSEPH DOUGLAS BRAILSFORD. ERNEST FREDERICK GOODENOUGH. 

